The purpose of this study is to evaluate differences in parasympathetic effects on adrenergic receptor function between hostile/Type A (coronary- prone) and nonhostile/Type B (noncoronary-prone) individuals. Our previous work has shown larger beta receptor-mediated electrophysiologic responses to isoproterenol infusion among hostile Type A's. We hypothesize that this effect is due to deficient vagal protection of the ventricular myocardium against sympathetic effects among hostile Type A's.